Temporal dynamics of Grapevine red blotch virus accumulation in grapevine leaves is influenced by fruit maturity stages.

Grapevine red blotch disease (GRBD) poses a serious threat to viticulture in the United States. Grapevine red blotch virus (GRBV), a member species of Grablovirus vitis (family Geminiviridae), the causal agent of GRBD, disrupts grapevine physiology and metabolism, thereby interfering with the natural processes of fruit ripening. To date, there is limited knowledge about how grape maturity stages influence the timing of changes in viral load and disease progression. This study elucidates the effect of fruit maturity stages on GRBV accumulation in Vitis vinifera L. cv. Merlot grapevines in a Central Coast vineyard in California. Petioles from six basal leaves at random from previously GRBV-infected vines were collected at pre-véraison, véraison, post-véraison, and harvest stages, across two years (2021 and 2022). The viral copy number was quantified using digital PCR. The study identified significant differences (p < 0.05) in GRBV copy number across different maturity stages in 2021, irrespective of the year of infection. The lowest viral load was observed during the pre-véraison stage, and the highest at harvest, indicating progressive viral accumulation as the grapevines development progressed. In 2022, however, no significant difference in viral load across maturity stages was detected, an outcome attributed to extreme heat spikes coinciding with sampling periods. Our findings highlight the interplay between GRBV accumulation, grape development, and environmental parameters.